MOSCOW, Dec 20 (Reuters) - President Vladimir Putin supported a ban on Americans adopting Russian children on Thursday in a feud over a U.S. human rights law which he said was poisoning relations.

Putin, 60, struck a hawkish tone in his first annual news conference since he began a new six-year term in May but denied that tough measures against his opponents since his return to the Kremlin meant he was an authoritarian leader.

He used the marathon live broadcast to dismiss speculation about his health, underline the strength of the economy and portray himself as the guarantor of stability in a country that was under Soviet communist rule two decades ago.

"This is by no means the least successful period in Russia's history," he said, adding: "Because I love Russia."

"Without irony, I look forward to any future president being more successful."

Sitting behind a large desk in front of 1,200 journalists in a Moscow conference centre, Putin remained calm, rarely smiled and was still going after three hours. But he became particularly animated when asked about the spat with Washington.

He described as "unfriendly" the legislation signed by President Barack Obama last week that will punish Russians accused of violating human rights by refusing them visas and freezing their assets in the United States.

"This, of course, poisons our relationship," he said of the measures, drawn up because of concern over the death in a Russian prison of anti-corruption lawyer Sergei Magnitsky in 2009.

The dispute threatens efforts by Putin and Obama to improve ties between the former Cold War enemies following their presidential election victories this year.

But Putin signaled that he would sign into law a tit-for-tat move proposed by Russia's lower house of parliament to prevent Americans adopting Russian children as well as barring entry to U.S. citizens accused of abusing Russians' rights.

"It is an emotional response by the State Duma but it is an appropriate response," he said of the parliamentary measure.

The Kremlin says Obama is likely to visit Russia in the first half of 2013 but Western diplomats say the U.S. president will agree to a summit only if he feels progress can be made.

Asked about the conflict in Syria, another irritant in relations with Western powers which have backed President Bashar al-Assad's enemies, Putin said Moscow's main concern was the fate of the country and not that of its long-time ally Assad.

He said Moscow wanted to ensure that any solution to the conflict in Syria must prevent the opposition and government forces just swapping roles and continuing to fight indefinitely:

"We are not concerned about the fate of Assad's regime. We understand what is going on there," Putin said. "We are worried about a different thing - what next."

PUTIN DENIES HEALTH CONCERNS

During his first spell as president from 2000 until 2008, Putin began a tradition of giving long annual news conferences to show his grasp of policy detail. The last one, in 2008, ran for four hours and 40 minutes.

He appeared intent on Thursday on showing he is strong and healthy and has a firm grip despite the biggest protests against him since he began his 13-year domination of Russia, the world's largest energy exporter.

The Kremlin has dismissed suggestions that Putin has serious health problems since he was seen limping at a September summit and Russian government sources told Reuters he was suffering from back trouble.

"This is only beneficial for political opponents who are trying to question the legitimacy and the effectiveness of the authorities," Putin said. "I can give the traditional answer to the health question: there's no point in waiting."

Critics, including in the United States and Europe, accuse him of trying to smother dissent by pushing through laws that they say can be used to stifle opponents.

But Putin said: "I cannot call it authoritarian, I cannot agree ... I think that order, discipline and following the rule of law do not contradict democracy."

Addressing questions after question, Putin showed he still has a command over detail. But opponents dismissed his remarks on democracy and suggested he was out of touch.

"'Because I love Russia.' But if you respect it too, why do you treat it like cattle?" tweeted broadcaster and commentator Yevgenia Albats.

Another Twitter user, who identified himself as Sergei Neptun, wrote: "Mr President, how much longer do we have to put up with this lawlessness in the country?"

ECONOMY IN GOOD SHAPE

Putin began the news conference by reeling off economic data for the world's ninth-largest economy, forecasting that it would grow by 3.7 percent this year.

"This is a good result overall," he said, suggesting that Russia's economy was performing well, particularly if it was compared with the euro zone and the United States.

Putin said recession in the euro zone had acted as a drag on Russian growth and that a poor harvest had hit the economy in the third and fourth quarters, lifting inflation over 6 percent.

He expressed concern over a slowdown in industrial output growth. But he highlighted Russia's low unemployment rate of 5.3-5.4 percent, which he described as "good - one of the best in all the developed economies of the world".

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